Just tried it in Mozilla 1.5, same problem, though it works fine in Netscape 4.8. It is curious that this is the only site that is having problems with Mozilla based browsers which indicates that it's a combination of things i.e. server plus browser.
The only thing I have been able to track down is that Gecko based browsers have a problem if the mime type of css files is set incorrectly by the server. Try putting an .htaccess file in your root directory (assuming Apache) with this
AddType text/css .css
Just done some more research on this. It appears this problem is fairly widespread and the world is littered with servers who have mine type set as text/plain. The problem is that in compliance mode Mozilla Firebird insists on the server mime type matching the document mime type in the webpage. If it doesn't then it will just ignore the css file. The quick way to test for this problem would be to change the doctype definition in main.tpl to one which would throw Firebird into quirks mode (or just remove the doctype alltogether). This problem does not affect IE or Opera because they are simply not very fussy about mime types.